1. How do LVIT position sensors work?
A LVIT (Linear Variable Inductance Transducer) is a type of linear position sensor. Similarly to the more common LVDT (Linear variable differential transformer) sensor, the LVIT uses the principal of electrical inductance to measure position. However, whereas the LVDT sensor induces an electric field with one coil and measures it with a second coil, the LVIT generates and measures the electric field using the same coil. This difference in operating principal, results in the LVIT sensor being significantly more compact than the LVDT sensor.
The LVIT consists of a single coil of copper wire wound around a static, non-metal shaft. An AC voltage passes through the coil*, inducing an electric field. A hollow iron cylinder, attached to the target object, passes over the copper coil. Due to the high magnetic permeability of the iron cylinder, the electric field preferentially runs through the iron cylinder. The presence of the iron cylinder around the coil acts as a load, resisting the flow of current through the coil and thereby causing a drop in voltage and a reduction in AC frequency (the reason for which is explained here) in proportion to the extent of the overlap between the coil and cylinder. The internal electronics of the LVIT sensor measures the AC frequency (it could measure the voltage, but this would be less accurate) of the current flowing through the coil, from which it determines the position of the cylinder.
*The LVIT requires a DC supply voltage. The AC voltage through the coil is formed by a capacitor placed in series with the inductance coil in what is known as an LC circuit. In fact, the entire principal of operation of the LVIT sensor (i.e. that the magnitude and frequency of the coil current is diminished by the presence of the iron cylinder) relies on the coil being part of an LC circuit and not being supplied from an external AC source.
2. General characteristics of LVIT position sensors
LVIT sensors are high accuracy sensors with full scale measurement ranges from 0.2 to 100 cm and the ability to withstand strong vibrations and harsh environments (typically rated to IP67 ingress protection). They are medium cost sensors and although they require relatively complicated electronics, the circuitry is built into the sensor. LVIT sensors are available with either guided or unguided rods, the latter enables an unlimited number of cycles without failure and a high linear velocity, limited only by sampling frequency (as velocity increases, the distance traveled between successive sensor readings increases).
3. Input and output signals
LVIT sensors are powered by a 5-30 Vdc supply voltage which is converted to an AC voltage by the sensors internal LC circuit. The internal electronics of the sensor measures the AC frequency of the coil voltage to determine the position of the cylinder and also performs temperature compensation. The sensor outputs a standard signal form such as 0-10 V or 4-20 mA.
4. Applications of linear LVIT position sensors
Since LVIT sensors possess similar performance characteristics to LVDTs, they are used in similar applications; namely those where the lower cost linear potentiometer is insufficient because of a requirement for higher accuracy, robustness or lifetime (cycles to failure). The LVIT is significantly more compact than the LVDT which makes it preferential for applications where size is important. Despite this advantage of the LVIT, the LVDT remains significantly more popular than the LVIT.
5. Typical specification
|Measurement range||0 to 100 cm|
|Velocity||Limited primarily by spatial resolution considerations|
|Lifetime (cycles)||very high, up to unlimited|
|Ambient temperature||-40 to 120°C|
|Supply voltage||5-30 Vdc|
|Output signal||0-10 V, 4-20 mA, mV/V/mm|
|Vibration resistance||10 g|
|Shock resistance||1,000 g|
|Ingress Protection||IP67 typical|
|Passive / active||Active|
|Contact / non-contact||Minimal or no contact|
6. Purchasing tips
- Spring loaded: Many LVIT sensors include spring loaded cylinders which do not need to be mechanically coupled to the target object because the spring ensures contact is always maintained. The other option is mechanical coupling through a rod end joint. Sensors which include rod end joints may also be spring loaded.
- Ball joint: Many LVIT sensors include a rod end ball joint at the end of the cylinder to help absorb miss-alignment between the cylinder and the target object. There are LVIT sensors available with an additional ball joint on the static sensor body to enable the entire sensor to pivot.
- Rod Configurations: LVIT sensors are available with either guided or unguided rods. Guided rods are guided by ball bearings so that they can only move axially. Unguided rods do not make any contact with the opposing side of the sensor. Instead, they rely on being connected to and guided by the object whose position is being tracked by the LVIT.
7. Advantages of LVIT sensors
- Have a high measurement accuracy, similar to that of LVDT sensors and superior to linear potentiometers. This is partly due to their built in temperature compensation and noise reduction.
- Are much more compact in size than LVDT sensors. In fact, a LVIT sensors is about half the length of a LVDT sensor of comparable full scale measurement range.
- Are powered by a DC supply voltage, which is easier to provide than the AC supply voltage required by LVDT sensors (unless the LVDT has an internal DC to AC converter).
- Are capable of withstand harsh environments and high amplitude vibrations (typically 10 g continuous vibration, 1000 g shock and IP67 ingress protection).
8. Disadvantages of LVIT sensors
- Are more expensive than potentiometer based linear position sensors. This is in part due to their more complex built in electronics. As such, their may be no advantage of choosing an LVIT sensor for a non-challenging application.
9. Application tips
- Non-axial force: Be careful not to apply non-axial force to the sensors hollow cylinder as this could damage the LVIT sensor. Rather than causing catastrophic failure (which would be easy to detect), the damage is likely to result in a deterioration of accuracy which can often cause more problems than catastrophic failure.
- High accelerations: If relying on a spring for maintaining contact with the target object, keep in mind that high accelerations of the target can cause temporary separation of the cylinder from the object. Stiffer springs provide better contact with the target.